1. Field of the Invention
The present invention relates to a toner for use in, for example, electrophotographic, electrostatic recording and magnetic recording technologies.
2. Description of the Related Art
Conventionally, in electrophotographic systems, an electrostatic latent image bearing member (referred to below as a “photosensitive member”) which is generally composed of a photoconductive material is charged by various means then exposed to light, thereby forming an electrostatic latent image on the surface of the photosensitive member. Next, the electrostatic latent image is developed with toner on a toner bearing member (referred to below as a “developing sleeve”) to form a toner image, and the toner image is transferred to a transfer material such as paper, following which the toner image is fixed on the transfer material by heat, pressure or the application of both heat and pressure, yielding a copied article or a print. At this time, the toner that has not been transferred to the transfer material and remains on the photosensitive member following transfer (untransferred toner) is cleaned off by various methods, and the above steps are repeated.
One known cleaning system is a blade cleaning method that mechanically removes untransferred toner by pressing an elastic rubber blade against the photosensitive member.
In recent years, the desire for higher speed, higher image quality and smaller equipment size in copiers and printers which use electrophotographic technology has created a need to increase the process speed of the apparatus while at the same time furnishing high-resolution images. However, the burden on the toner increases at higher speeds, and problems relating to development performance, such as a decline in the image density caused by toner deterioration, have a tendency to arise.
Moreover, in the cleaning step, increasing the process speed of the apparatus makes it difficult for the cleaning blade to properly scrape away the toner, and allows toner to pass by the cleaning blade. As a result, what is referred to as “faulty cleaning” tends to arise.
A key technology in downsizing copiers and printers is to reduce the size of the developing sleeve. The application of charge to the toner is carried out by triboelectric charging due to rubbing between the toner and a triboelectric charge-providing member such as the developing sleeve in a region where the toner has been regulated primarily by a toner regulating member (referred to below as the “developing blade”).
In the case of a smaller developing sleeve in particular, the developing zone of the development nip becomes smaller, making it more difficult for toner to jump from the developing sleeve. As a result, the phenomenon known as “charge-up” occurs in which only a portion of the toner becomes excessively charged, sometimes causing various image defects.
For example, the charged up toner remains on the developing sleeve, leading to a decrease in image density and making charging of the toner non-uniform, as a result of which image defects such as fogging in non-image regions sometimes arises.
In addition, the charged up toner tends to adhere strongly to the photosensitive member, making it difficult to remove in the cleaning step, which readily leads to faulty cleaning. Also, such toner has a tendency to pack tightly at the back of the cleaning blade, as a result of which the untransferred toner is not completely recovered, readily giving rise to the problem of waste toner spillage. Such problems can become quite serious, particularly in low-temperature, low-humidity environments where which toner charge-up readily occurs.
One method for improving the cleaning performance is to increase the pressure of the cleaning blade against the photosensitive member. However, simply increasing the blade pressure tends instead to give rise to such problems as vibration and curling of the cleaning blade. Also, from the standpoint of energy conservation, a low torque is preferred, and there are cases where a lower cleaning blade pressure is in fact preferred. Also, from a downsizing standpoint, because making the photosensitive member smaller increases the curvature at the surface of the photosensitive member, stable scraping with the cleaning blade becomes more difficult to achieve.
Toners in which an inorganic fine powder is externally added to the toner particles as an abrasive or a lubricant in order to improve the toner cleaning performance have also been proposed.
Japanese Patent No. 3385860 describes a toner obtained by the external addition to toner particles of strontium titanate fine particles that are sintered aggregates of primary particles having an average primary particle size of 30 to 150 nm.
However, with increasingly fine toner particles targeted at higher image quality, it becomes more difficult to obtain a stable image density. Moreover, because the state of attachment by silica and other inorganic fine particles is not controlled, this approach has not led to an improvement in cleaning performance within low-temperature, low-humidity environments.
When the diameter of a developing sleeve is made smaller, as mentioned above, charged up toner readily forms and toner charging tends to become uneven. In order for proper triboelectric charging of the overall toner to occur, toner circulation needs to take place in the region where rubbing with the developing sleeve and the developing blade is carried out (referred to below as the “blade nip”); that is, toner in contact with the developing sleeve or the developing blade must be replaced with toner that is not in contact. However, deteriorated toner has a poor ability to circulate, and so proper triboelectric charging of the overall toner tends to be difficult.
A great deal of research aimed at suppressing toner deterioration has hitherto been carried out.
Japanese Patent Application Laid-open No. 2009-186812 describes an emulsification aggregation toner for which the ratio of free large-particle-size silica (free ratio) has been specified. Japanese Patent Application Laid-open Nos. 2008-276005, 2010-60768 and 2009-229785 all describe technology for enabling toner to withstand long-term use by improving the attached state of the external additive and thereby altering toner flowability.
Such related art has indeed provided a certain degree of advantageous effects in terms of stability when used in durability tests and in terms of the cleaning performance. Yet, in cases where, as described above, the diameter of the developing sleeve has been made smaller, and also in low-temperature environments, satisfactory solutions have not been developed, leaving room for further improvement.